How to decode Flutter iso8601 DateTime to Go Api RFC3339 pgtype.Date and pgtype.Timestamptz

Go uses RFC 3339 for encoding time, if you control the json being produced you just need to change 2022-04-03T00:00:00.000 to 2022-04-03T00:00:00.000Z.

For instance this works.

type Contract struct {
    ContractId          *int       `json:"contract_id"`
    CompanyId           *int       `json:"company_id"`
    DateEstablished     *time.Time `json:"date_established"`
    ExpiryDate          *time.Time `json:"expiry_date"`
    ExtensionExpiryDate *time.Time `json:"extension_expiry_date"`
    Description         *string    `json:"description"`
}

func main() {
    body := `{"contract_id":0,"date_established":"2022-04-03T00:00:00.000Z","expiry_date":null,"extension_expiry_date":null,"description":"fffff"}`

    var contract Contract
    reader := strings.NewReader(body)
    decoder := json.NewDecoder(reader)
    err := decoder.Decode(&contract)
    if err != nil {
        fmt.Println("Error: ", err)
    } else {
        fmt.Printf("Contract: %+v\n", contract)
    }
}

If you don't control the json, you need to write a custom unmarshal method.

This is the solution I finally implemented in Flutter using pgtype.Date and pgtype.DateTimestamptz in my Go api and column types DATE and DATETIME in Postgresql. I was unable to use a pointer with this data type.

First I created a helper in Flutter like this:

import 'package:intl/date_symbol_data_local.dart';
import 'package:intl/intl.dart';

extension DateTimeExtension on DateTime {
  String format([String pattern = 'dd/MM/yyyy', String? locale]) {
    if (locale != null && locale.isNotEmpty) {
      initializeDateFormatting(locale);
    }
    return DateFormat(pattern, locale).format(this);
  }

  String formatDbDateTime(
      [String pattern = "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'", String? locale]) {
    if (locale != null && locale.isNotEmpty) {
      initializeDateFormatting(locale);
    }
    return DateFormat(pattern, locale).format(this);
  }

  String formatDbDate([String pattern = 'yyyy-MM-dd', String? locale]) {
    if (locale != null && locale.isNotEmpty) {
      initializeDateFormatting(locale);
    }
    return DateFormat(pattern, locale).format(this);
  }

  String formatLocalDate([String pattern = 'dd MMMM yyyy', String? locale]) {
    if (locale != null && locale.isNotEmpty) {
      initializeDateFormatting(locale);
    }
    return DateFormat(pattern, locale).format(this);
  }

  String formatLocalDateTime(
      [String pattern = 'dd MMMM yyyy h:mm a', String? locale]) {
    if (locale != null && locale.isNotEmpty) {
      initializeDateFormatting(locale);
    }
    return DateFormat(pattern, locale).format(this);
  }
}

I used this in my Flutter models to format the date for my Go Api:

if (planAcceptedDate != null) {
  data['plan_accepted_date'] = planAcceptedDate!.formatDbDateTime();
} else {
  data['plan_accepted_date'] = null;
}

if (invoiceDate != null) {
  data['invoice_date'] = invoiceDate!.formatDbDate();
} else {
  data['invoice_date'] = null;
}

and in my Flutter view form I use it like this:

        Container(
          padding: EdgeInsets.all(8.0),
          child: Text(
            _matter.planAcceptedDate == null
                ? ''
                : _matter.planAcceptedDate!.formatLocalDateTime(),
            style: TextStyle(
              color: appTextColor,
              fontWeight: FontWeight.normal,
              fontSize: _user.fontsize,
            ),
          ),
        ),
        Container(
          padding: EdgeInsets.all(8.0),
          child: Text(
            _matter.invoiceDate == null
                ? ''
                : _matter.invoiceDate!.formatLocalDate(),
            style: TextStyle(
              color: appTextColor,
              fontWeight: FontWeight.normal,
              fontSize: _user.fontsize,
            ),
          ),
        ),

My Go struct looks like this:

PlanAcceptedDate pgtype.Timestamptz `json:"plan_accepted_date"`
InvoiceDate      pgtype.Date        `json:"invoice_date"`